Recent decline in hepatitis E virus prevalence among wild boars in Japan: Probably due to countermeasures implemented in response to outbreaks of classical swine fever virus infection

•HEV prevalence and genetic diversity were studied in 16 prefectures from 2018-2023.•Anti-HEV IgG was found in 10.7 % of boars; HEV RNA was found in 3.6 % of boars.•HEV-3 was most common (91.9 %), with HEV-4 and a HEV-6-related strain also present.•HEV prevalence in wild boars decreased significantly from 2018/2019 to 2022/2023.•Decline in HEV among boars may be linked to classical swine fever virus outbreaks. Previous studies have emphasized the necessity of surveillance and control measures for hepatitis E virus (HEV) infection in wild boars, an important reservoir of HEV. To assess the current situation of HEV infection in wild boars in Japan, this study investigated the prevalence and genetic diversity of HEV among wild boars captured in 16 prefectures of Japan during 2018–2023. Serum samples from 968 wild boars were examined for anti-HEV IgG antibodies and HEV RNA. The prevalence of anti-HEV IgG varied geographically from 0 % to 35.0 %. HEV RNA was detected in 3.6 % of boars, with prevalence varying by prefecture from 0 % to 22.2 %. Genotype 3 was the most prevalent genotype (91.9 %), followed by genotype 4 (5.4 %), with one strain closely related to genotype 6. The prevalence of HEV infection among wild boars decreased from 2018/2019 to 2022/2023 with significant declines in levels of anti-HEV IgG antibodies (14.5 % vs. 6.2 %, P < 0.0001) and HEV RNA (7.6 % vs. 1.5 %, P < 0.0001). Regional analysis showed varying trends, with no HEV RNA-positive boars found in several regions in recent years. A plausible factor contributing to the decline in HEV infection is the application of countermeasures, including installing fences to prevent intrusion into pig farms, implemented in response to the emergence of classical swine fever virus (CSFV) infection in wild boars and domestic pigs, with incidents reported annually since 2018. Further investigation is warranted to explore the association between countermeasures to CSFV infection and the decrease in HEV infection among wild boars.